Enantioselective catalysis using chiral metal complexes provides one of the most general and flexible methods for achieving asymmetric organic reactions. Metallic elements possess a variety of catalytic activities, and permutations of organic ligands or other auxiliary groups directing the steric course of the reaction are practically unlimited. Efficient ligands must be endowed with, for example, suitable functionality, an appropriate element of symmetry, substituents capable of differentiating space either electronically or sterically and skeletal rigidity or flexibility.
Among the asymmetric organic reactions catalyzed by chiral transition metal complexes, asymmetric hydrogenation has been one of the best studied, due in large part to the fact that it is the basis for the first commercialized catalytic asymmetric process. See, for example, ApSimon, et al., Tetrahedron, 1986, 42, 5157.
Some of the more interesting of the asymmetric hydrogenation catalysts are those derived from BINAP [2,2'-bis(diphenyl-phosphino)- 1,1'-binaphthyl]. See, for example, U.S. Pat. Nos.: 4,691,037; 4,739,084; 4,739,085; and 4,766,227. Unlike the more classical models of chiral (asymmetric) molecules, chirality in the case of the BINAP compounds arises from the restricted rotation about the single bond joining the naphthalene, i rings. Because of such restricted rotation, perpendicular disymmetric planes result. Isomers arising from this type of asymmetry are termed atropisomers.
Cationic rhodium-BINAP complexes have been shown to catalyze the isomerization of allylamines to chiral enamines in 94-96% ee. Also, hydrogenations of geraniol and nerol (bis-unsaturated alcohols) using rhodium-BINAP complexes produce products in about 50% eels. The synthesis of BINAP derivatives bearing groups other than phenyl on phosphorus such as paramethylphenyl and cyclohexyl have also been prepared. Inoue, et al., Chem. Lett., 1985, 1007.
Studies on the mechanism of rhodium-phosphine catalyzed asymmetric reductions of .alpha.,.beta.-unsaturated acids or esters bearing an .alpha.-acetamido group have shown that the reaction proceeds by the displacement of solvent by the unsaturated substrate forming a chelate complex in which the olefin and the carbonyl oxygen of the acetamido function are bound to the metal. See Halpern, J., Asymmetric Synthesis, Vol. 5, pp. 41-69, J. D. Morrison, Ed., Academic Press, Inc., 1985. Substrates lacking the .alpha.-acetamido group are reduced with far less stereoselectivity. .alpha.,.beta. and .beta.,.gamma.-unsaturated amides similarly form complexes in which the olefin and carboxamide oxygen are bound to rhodium. These reactions proceed with high stereoselectivity. See Brown, et al, J. Org. Chem., 47, 2722 (1982) and Koenig, K. E., Asymmetric Synthesis, Vol. 5, pp. 71-101, J. D. Morrison, Ed., Academic Press, Inc., 1985.
The BINAP ruthenium complexes are dramatically different than the rhodium ones. They have been used to catalyze a variety of asymmetric hydrogenations, including the hydrogenation of enamides and alkyl and aryl-substituted acrylic acids. See Noyori, et al., Modern Synthetic Methods, 1989, 5, 115, incorporated herein by reference.
However, unlike the rhodium catalyzed reductions, ruthenium (II) carboxylate complexes possessing the BINAP ligand are efficient catalysts for the enantioselective transformation of .alpha.,.beta.-unsaturated carboxylic acids. Amide-bearing olefins as well as carboxylic acid esters are essentially unreactive with these catalysts. According to Ohta, et al, J. Org. Chem, 52, 3174 (1982), the carboxylate moiety, and not other oxygen containing groups, is responsible for the stereoselective reaction. Noncarboxylate-containing substrates are unaffected by ruthenium complexes in these asymmetric reductions.